The present invention relates to a method of lowering the forward current gain of an inherent bipolar transistor in a semiconductor device such as an insulated-gate transistor or an insulated-gate field effect transistor, and further relates to semiconductor devices resulting from the present gain reduction method.
A typical insulated-gate transistor (IGT) constitutes a 4-layer semiconductor device (i.e., a P-N-P-N device). The 4-layer structure constitutes an inherent thyristor in the IGT and is susceptible to latching into an "on", or conducting, state in thyristor fashion, in which case control of IGT current by the gate thereof is lost and turn-off of the IGT requires reversal of the voltage impressed across the device, a technique known as commutation. It is known that a 4-layer device may be modelled as a pair of regeneratively coupled bipolar transistors. It is further known that thyristor action will be suppressed in the 4-layer device so long as the sum of the forward current gains of the two, inherent bipolar transistors is maintained below unity. The present invention is particularly directed towards reduction in the forward current gain of one of the inherent bipolar transistors in a 4-layer device, such as an IGT, with the ultimate goal of increasing the level of current that an IGT can conduct before latching on in thyristor fashion.
A typical insulated-gate field-effect transistor (IGFET), commonly referred to as power metal-oxide-semiconductor field-effect transistor (MOSFET), also includes an inherent bipolar transistor, turn-on of which is desirably suppressed for IGFET operation at high current levels. This can be achieved by the method of the present invention, which is directed towards a reduction in the forward current gain of such an inherent bipolar transistor.
It, accordingly, is a principal object of the invention to provide a method for reducing the forward current gain of an inherent bipolar transistor in an insulated-gate semiconductor device and also to provide the resulting devices.
It is a further object of the invention to provide a method of reducing the forward current gain of an inherent bipolar transistor in an insulated-gate semiconductor device wherein the method may be carried out through the use of conventional semiconductor fabrication equipment.
A more particular object of the invention is to provide an insulated-gate transistor with increased current-conduction capability without thyristor latchon.
In an embodiment of the invention particularly concerning an IGT, the foregoing objects are attained in a method of reducing the forward current gain of an inherent bipolar transistor in such device. The IGT is of the type including, in a silicon semiconductor wafer, a drift layer, a base layer overlying the drift layer, and an emitter layer overlying the base layer and forming an emitter-base junction therewith, the drift, base, and emitter layers constituting an inherent bipolar transistor. The IGT further includes a base electrode insulatingly spaced from the base layer.
The present method comprises the steps of providing a source of ions having the capacity to create current carrier recombination centers in silicon semiconductor material upon implantation therein. These ions are implanted into the upper portion of the silicon semiconductor wafer at an energy level selected to yield a layer of ions with a peak concentration situated at the emitter-base junction, within a tolerance of about plus or minus 20 percent of the thickness of the emitter layer. The implant dosage of the ions is preferably selected to be within a range having a lower limit (1) that is sufficiently high so as to reduce by at least about 1 order of magnitude the forward current gain of the inherent transistor and having an upper limit (2) that is sufficiently low as to raise by no more than about 20 percent the respective sheet resistivities of whichever of the emitter and base layers wherein the ion layer is situated.
In accordance with the invention, IGTs produced by the foregoing method are also provided.